hydrogeological & geophysical investigations · hydrogeology and existing borehole data, for...

Hydrogeological & Geophysical Investigations

Igara African Devine Church,

Walatsi Location, Nambale District

Busia County

***********

September, 2012

CONSULTANT:

Sebastian Namwamba

(Registered Hydrogeologist; Licence No. WD/WP/32)

EARTHS SCOPE-GEO HYDRO SERVICES

Water, Sanitation and Environmental Engineering

P. O. Box 17783-00100

NAIROBI

Cell-phone: 0720-438377

Signed: Date

Report No. 42/2012

HHyyddrrooggeeoollooggiiccaall && GGeeoopphhyyssiiccaall IInnvveessttiiggaattiioonnss AAffrriiccaann DDeevviinnee CChhuurrcchh IIggaarraa,, BBuussiiaa CCoouunnttyy,, KKeennyyaa..

EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee ii

SUMMARY

Introduction

This report presents the results of hydrogeological and geophysical investigations to

evaluate groundwater resources for Igara Devine Church in Walatsi Location, Nambale

District of Busia County, Kenya.

Climate

The climate in the area is mainly humid type, with a bi-modal rainfall pattern. Long rains

are experienced in March to July while short rains occur in August to October. The main

source of rainfall over Igara region is the Lake Victoria. It occurs in form of either

convectional or frontal rainfall. The mean annual rainfall ranges between 1100 and 2700

mm. The mean maximum and minimum temperatures in the area are given as 27oC and

15oC respectively. Evaporation is relatively low and increases from less than 1400 mm to

1800 mm per year.

Geology

The regional geology of the investigated area is dominated by the Basement System

Complex which can be divided into two groups: - (a) The Basement Complex rocks,

(sedimentary in origin) and the (b) The undeformed post- Basement formations. The

geology of the investigated site include, the recent deposits which include lateric iron

stone resulting to production of thick red soils. These are underlain by the Kavirondian

series rocks consisting of feldspathic grits, conglomerates, shale and siltstones. Granites

are the major intrusives in the area.

Geophysical Fieldwork

Geophysical fieldwork was executed on 30th

September 2012. The resistivity method was

used for the present investigations. Geophysical measurements were used to determine the

thickness of the underlying layers, their potential as aquifers, and the expected quality of

groundwater in these formations .One vertical electrical soundings were executed on

selected points

Conclusions

From the present study it is concluded that the prospects for sufficient groundwater for use

at the chuch and the neighboring community are good, based on the geological and

hydrogeological evidence gathered.

Recommendations for Drilling

Excavation of a shallow well is recommended for this area though a borehole would be

more fissible to supply the big population in this area.

Alternatively drilling of a Borehole to a maximum 60 m was recommended in this church

compound. The site was shown to Mr. William Ashitiva-0710418718, George Wafula-

07156692 and Ms Mary Opech-0725950574 an elder in the area.

The borehole should be drilled at 8‟‟ diameter and installed with 6" diameter uPVC casings

and screens with high % open surface area. Recommendations are given for borehole

construction and completion methods. The importance of correct and comprehensive

techniques in this particular aspect cannot be over-emphasized. Generally, the water

quality is expected to be good.


EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee iiii

Monitoring

Regular monitoring should be instituted and maintained in the borehole in order to keep

track of groundwater levels. A piezometer should be installed in the borehole to be able to

monitor the water level in the well.

Drilling

A groundwater drilling permit will have to be obtained from the Water Apportionment

Board of the Ministry of Water and Irrigation.

Additional recommendations for borehole drilling, installation, development and

testpumping are given in Appendix 1

.


EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee iiiiii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................................................... 1

1.1. BACKGROUND ............................................................................................................................................. 1 1.2. SCOPE OF THE WORKS ................................................................................................................................ 1 1.3. REPORTING REQUIREMENTS ........................................................................................................................ 2

2. BACKGROUND INFORMATION ................................................................................................................... 3

2.1. LOCATION ................................................................................................................................................... 3 2.2. VEGETATION AND CLIMATE ........................................................................................................................ 3 2.3. TOPOGRAPHY AND DRAINAGE .................................................................................................................... 3 2.4. EXISTING WATER SUPPLY AND DEMAND .................................................................................................... 3

3. GEOLOGY .......................................................................................................................................................... 5

3.1. REGIONAL GEOLOGY .................................................................................................................................. 5 3.1.1. Precambrian .......................................................................................................................................... 5 3.1.2. Cretaceous ............................................................................................................................................. 5 3.1.3. Tertiary .................................................................................................................................................. 5 3.1.4. Pleistocene to Recent............................................................................................................................. 6

3.2. GEOLOGICAL ROCK FORMATIONS ............................................................................................................... 6 3.2.1. Pleistocene and Recent Deposits ........................................................................................................... 6 3.2.2. Older Rocks ........................................................................................................................................... 6 3.2.3. Kavirondian System ............................................................................................................................... 6 3.2.4. Nyanzian System .................................................................................................................................... 7 3.2.5. Basement System ................................................................................................................................... 7

3.3. GEOLOGY OF THE INVESTIGATED SITE ........................................................................................................ 7

4. HYDROGEOLOGY ........................................................................................................................................... 9

4.1. GROUNDWATER OCCURRENCE ................................................................................................................... 9 4.2. BASEMENT AQUIFER OCCURRENCE ............................................................................................................ 9 4.3. WEATHERING AND ITS SIGNIFICANCE IN AQUIFER OCCURRENCE .............................................................. 10

4.3.1. The Collapsed Zone ............................................................................................................................. 10 4.3.2. Saprolite .............................................................................................................................................. 10 4.3.3. Saprock ................................................................................................................................................ 10 4.3.4. Bedrock ............................................................................................................................................... 10 4.3.5. Regolith ............................................................................................................................................... 11

4.4. HYDROGEOLOGY OF THE IGARA AREA ...................................................................................................... 11 4.5. BOREHOLE DATA ...................................................................................................................................... 12 4.6. GROUNDWATER QUALITY ......................................................................................................................... 12

5. GEOPHYSICAL INVESTIGATION METHODS ......................................................................................... 13

5.1. RESISTIVITY METHOD ............................................................................................................................... 13 5.2. BASIC PRINCIPLES ..................................................................................................................................... 13 5.3. VERTICAL ELECTRICAL SOUNDING (VES) ................................................................................................ 14

6. FIELDWORK AND RESULTS ....................................................................................................................... 15

6.1. FIELDWORK ............................................................................................................................................... 15 6.2. INTERPRETATION RESULTS ....................................................................................................................... 15

7. CONCLUSIONS AND RECOMMENDATIONS .......................................................................................... 17

7.1. CONCLUSIONS FIELDWORK ....................................................................................................................... 17 7.2. RECOMMENDATIONS ................................................................................................................................. 17

8. REFERENCES .................................................................................................................................................. 18


EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee iivv

FIGURES

Figure 1:Site and Borehole/Shallow Well location map of the Investigated Area ................................................. 4 Figure 2: Geological Map of the Study Area (After Beicip- Malmaison – France 1987) ...................................... 8 Figure 3:Vertical Electrical Sounding Interpretation for Igara VES 1 ................................................................ 16

TABLES

Table 1: Borehole-C7011 .......................................................................................................................................... 12 Table 2: GPS Co-ordinates of the VES Location.................................................................................................... 15 Table 3: Interpretation Results for Igara Devine Church ..................................................................................... 15 Table 4: Interpretation Results for Igara Devine Church ..................................................................................... 15

AAPPPPEENNDDIICCEESS

Appendix 1:Drilling ................................................................................................................................................... 20


EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee vv

LIST OF ABBREVIATIONS AND GLOSSARY OF TERMS

ABBREVIATIONS: (NOTE: SI spellings used throughout).

EC Electrical Conductivity (in micro Seimens/centimetre)

km kilometres

m metres

m amsl metres above mean sea level

m bgl metres below ground level

ppm parts per million, equivalent to mg/l

swl static water level (in m bgl) (the piezometric level or water table, see

below)

TDS Total Dissolved Solids (ppm)

wsl water struck level (in m bgl)

GLOSSARY OF TERMS:

Aquifer A geological formation or structure which transmits water

and which may supply water to wells, boreholes or springs.

Confined Confined aquifers are those in which the piezometric level is higher

(i.e., at a greater elevation relative to sea level) than the elevation at

which the aquifer was encountered.

Intercalated Interbedded - a lava flow may occur between layers of sediment, or

vice-versa.

Recharge The general term indicating the process of transport of water from

surface sources (i.e., from rivers or rainfall) to groundwater storage.

Volcanics Here used as a general term describing geological material of

volcanic origin.


EEaarrtthhss SSccooppee GGeeoo--HHyyddrroo SSeerrvviicceess OOccttoobbeerr 22001122,, PPaaggee 11

11.. IINNTTRROODDUUCCTTIIOONN

11..11.. BBaacckkggrroouunndd

Earths Scope Geo- Hydro Services Water Ltd. was commissioned by African Devine

Church to carry out borehole/Shallow well site investigations at their church in Walatsi

Location, Igara area of Nambale District, Busia County in WATSAN Program. The

fieldwork was carried out from 30th

September 2012.

The Client requires detailed information on prospects of drilling production Shallow well/

boreholes. The objective of the present study is to assess the availability of groundwater,

to recommend borehole drilling sites and comment on aspects of depth to potential

aquifers, aquifer availability and type, possible yields and water quality. For this purpose

all available hydrogeological information of the areas have been analyzed, and a

geophysical surveys done.

The investigations involved hydrogeological, geophysical field investigations and a detailed

desk study in which the available relevant geological and hydrogeological data were

collected, analyzed, collated and evaluated within the context of the Client's requirements.

The data sources consulted were mainly in four categories:

a) Published Master Plans.

b) Geological and Hydrogeological Reports and Maps.

c) Ministry of Water and Irrigation Borehole Completion records.

d) Technical reports of the area by various organizations.

11..22.. SSccooppee ooff tthhee WWoorrkkss

The scope of works includes:

(i) Site visits to familiarize with the project areas. Identify any issues that might

hinder the implementation of works in any of the areas and report to the Head of

Groundwater Investigation in the Ministry.

(ii) To obtain, study and synthesize background information including the geology,

hydrogeology and existing borehole data, for the purpose of improving the quality

of assessment and preparing comprehensive hydrogeological reports,

(iii) To carry out hydrogeological evaluation and geophysical investigations in

the selected sites in order to determine potential for groundwater and

appropriateness of drilling boreholes at the sites.

(iv) To prepare hydrogeological survey reports in conformity with the provisions of

the rules and procedure outlined by the Water Resources Management Authority,

including the following:

Site Name, Location and GPS readings

Geology and hydrogeology

Present sources and status of the existing water supply

Existing borehole data information.

Geophysical data and analysis



Conclusions and recommendations, including the groundwater potential of

the investigated sites, name and location of the site recommended for drilling,

recommended maximum drilling depth in metres and appropriate drilling

method

11..33.. RReeppoorrttiinngg RReeqquuiirreemmeennttss

The format of writing the Hydrogeological Investigations Report, as described out in the

Second Schedule of the Water Resources Management Rules, 2007. Such a report must

consider the following (verbatim): -

1. Name and details of applicant

2. Location and description of proposed Activity

3. Details of climate

4. Details of geology and hydrogeology

5. Details of neighbouring boreholes, including location, distance from proposed

borehole or boreholes, number and construction details, age, current status and

use, current abstraction and use.

6. Description and details (including raw and processed data) of prospecting

methods adopted, e.g. remote sensing, geophysics, geological and or

hydrogeological cross sections. Hydrogeological characteristics and analysis, to

include but not necessarily be limited to, the following:

a. Aquifer transmissivity

b. Borehole specific capacities

c. Storage coefficient and or specific yield

d. Hydraulic conductivity

e. Groundwater flux

f. Estimated mean annual recharge, and sensitivity to external factors

7. Assessment of water quality and potential infringement of National standards

8. Assessment of availability of groundwater

9. Analysis of the reserve

10. Impact of proposed activity on aquifer, water quality, other abstractors,

including likelihood of coalescing cones of depression and implications for

other groundwater users in any potentially impacted areas

11. Recommendations for borehole development, to include but not limited to, the

following:

a. Locations of recommended borehole(s) expressed as a coordinate(s)

and indicated on a sketch map

b. Recommendations regarding borehole or well density and minimum

spacing in the project area

c. Recommended depth and maximum diameter

d. Recommended construction characteristics, e.g. wire-wound screen,

grouting depth

e. Anticipated yield

12. Any other relevant information (e.g. need to monitor neighbouring boreholes

during tests).

This report is written so as to cover each of the above, insofar as data limitations allow.

The report also includes maps, diagrams, tables and appendices as appropriate.



22.. BBAACCKKGGRROOUUNNDD IINNFFOORRMMAATTIIOONN

22..11.. LLooccaattiioonn

The site is located in Igara area, along the Lupida-Mungatsi road in the western part of the

country in Busia County. The approximate location of the site is defined by co-ordinates

36 N 0651558/UTM 0058356. The location of the site is shown on Figure 1.

22..22.. VVeeggeettaattiioonn aanndd CClliimmaattee

The vegetation in the area is natural moist forest vegetation, comprising riverine

vegetation along rivers and thick bushes and woodland with dense under growth in the

undulating plains. Agriculture is extensive and has led to depletion of natural forest.

Several cash crop are grown in the area, the major cash crop being sugarcane. Others,

include maize, bean, sorghum and cassava. The area is generally humid, with average

annual rainfall in the range 1100-2700 mm. The mean maximum and minimum

temperature are in the range of 26-28 o

C and 14-16 oC respectively. The area is generally

classified as fairly warm.

22..33.. TTooppooggrraapphhyy aanndd DDrraaiinnaaggee

The area is generally undulating and characterized by broad flat topped divides and long

gentle valley slopes. Over most of the area the altitude ranges between 1400 - 1470 a msl.

The area is bounded on one side by one tributary draining into river Nzoia. The Nzoia

river is confined to the out crop of the Kisama - Igara Granite. While its valley is a mature

one, the river flows over a rocky bed which is undergoing excavation at the present day.

22..44.. EExxiissttiinngg WWaatteerr SSuuppppllyy aanndd DDeemmaanndd

Currently the Igara area has one borehole which was drilled by KEFINCO with numerous

shallow wells in the vicinity at a radius of 2 km.

The water demand for the community around Igara is about 10 m3 per day.



Figure 1: Site and Borehole/Shallow Well location map of the Investigated Area

Legend

Major Towns

Investigated Area

Rivers

Minor Roads

Major Roads



33.. GGEEOOLLOOGGYY

33..11.. RReeggiioonnaall GGeeoollooggyy

The regional geology can be found in GIBSON A.B (1954), report No. 26. The geological

history of the area can be followed from the Precambrian era, which is represented by the

metamorphic rocks of the „Basement System. The regional metamorphism was followed

by various cycles of erosion, faulting and graben formation, deposition of continental

sediments and volcanic activity.

33..11..11.. PPrreeccaammbbrriiaann

During this period, vast successions of sediments accumulated in a geosynclinal structure

that covered most of the present Central and Eastern Africa. Towards the end of the era, a

period of regional folding and metamorphism occurred. This large-scale tectonic activity

subjected the original rocks to high temperatures and pressures, which caused partial

melting. This was followed by re-crystallization and growth of new minerals. Depending

on the parent material and the prevailing pressures and temperatures, various types of

granites, gneisses and schists were formed.

Due to the tectonic events that followed their creation, the metamorphosed rocks were

intensely folded, tilted, and dissected by intrusive granitic dykes and domes. In addition,

granitization processes resulted in the formation of granulites and migmatites.

33..11..22.. CCrreettaacceeoouuss

During the Cretaceous, there existed a peneplain which is represented by a chain of high

points of uniform elevation in the Nandi highlands (Osorongai T.S., 7,006 ft.; Sumayat,

7,005 ft.), and continues south of the present area at similar level .The age of this

peneplain is thought to be Cretaceous and it has been correlated with the Kisii Highlands

peneplain some 160 km further south. This was followed by along period of erosion and

emplacement of minor intrusion

33..11..33.. TTeerrttiiaarryy

This is the period of Mt Elgon volcanic eruption, at the time of commencement of activity

the Basement surface possessed more or less the present characters. The volcanics of

mount Elgon never spread very far south or east beyond its present position. Below the

volcanic lavas were the sediments named the Bugishu series believed to be of Middle

Tertiary.



33..11..44.. PPlleeiissttoocceennee ttoo RReecceenntt

This period was dominated by further evolution of the majority of the rivers. These areas

are now on or below the level of the End-Tertiary Peneplain which lies some 100 to 190

m below the sub Miocene surface. Its presence is suggested by the plateau gravels,

consisting of well rounded quartz pebbles further away from the river, prominent

superficial deposits from weathered and eroded rock surfaces. Black valley soils and

lateritic soils are good example of the recent formations.

33..22.. GGeeoollooggiiccaall RRoocckk ffoorrmmaattiioonnss

There are three main rock formations that have been identified in the study area. These

are described in chronological order here below.

33..22..11.. PPlleeiissttoocceennee aanndd RReecceenntt DDeeppoossiittss

These are presented by river terraces and plateau gravels, black valley soils, laterite and

an often deep soil cover. River deposits consist of broad belts of marshy black, valley

soils and occur in the valleys of many rivers. This type of river deposits is especially the

characteristic of the Kavirondian terrain. In Basement area small alluvial sand and gravel

deposits are more common. More so the terrace gravel consisting of ungraded, rounded,

water-worn pebbles up to 6 inch diameter, mostly of agglomeratic type but with some

derived from granitic and the Basement gneiss might be found along some rivers e.g.

river Malakisi. The laterites is widespread and occur on all the rock type examined. The

quartz gravel of 1 foot thick band floats occurs at varying depth mostly over the north –

east of the area below the brown and red sandy soils. A deep soil cover has formed over

much of the area. The decomposition of Tertiary volcanics produces a rich dark red-

brown soils regarded best for agricultural purposes. The remainder of the area is covered

by somewhat lighter coloured sandy soils on the peneplain, with exception of the area of

gneissose leuco-granite which has a cover of highly quartzose acid soil but little removed

from freshly disintegrated rock and of little agricultural value.

33..22..22.. OOllddeerr RRoocckkss

Underlying the Pleistocene and Recent deposits, are the older rocks divided into two:- (a)

the Kavirondian System and (b) Nyanzian System.

33..22..33.. KKaavviirroonnddiiaann SSyysstteemm

The Kavirondian rocks are poorly exposed except in the valley of river Nzoia. In both the

arenaceous and the argillaceous members faint banding is recognizable at times, graded

bedding is sometime apparent and on occasion current –bedding also. Owing to the

proximity of granitic contacts to the sediments of the Nzoia valley, they have seldom

escaped contact metamorphic effects, and the development of the flakes of red brown

biotite is common. The so called “talcy” clays of the Bungoma area are derived by the

break down of Kavirondian rocks.



33..22..44.. NNyyaannzziiaann SSyysstteemm

Rocks of the Nyanzian System are exposed over a relatively small area and consist of

sheared, but otherwise unaltered, rhyollite with some small tuff. The main outcrop occurs

as a narrow band bounded by faults, on or near the crest of the Nandi escarpment and

continues into the area to the south. This band continues as numerous lenticular

discontinuous thrust sheets in the area of the Broderick Falls area. Nyanzian rocks or their

equivalents have not previously been found in contact with rocks of the Basement System

in Kenya or Tanzania.

33..22..55.. BBaasseemmeenntt SSyysstteemm

The Basement System rocks of the area do not show the diversity of high grade

metamorphic type which characterizes other areas of Basement System rocks surveyed in

Kenya. They consist of schist gneiss and migmatites derived from an original sedimentary

succession, which has been transformed by regional metamorphism and recrystallisation

during orogenic earth movements into foliated quartz and feldspar rich rocks with

variable amount of biotite and/or hornblende, locally abundant epidote and more rarely

garnet. Originally pure qurtzose sand stone now occur as granular quartzites, with small

amounts of muscovites, and originally less pure sandstone are now presented by quartz-

mica schists. Hornblende schists and garnetiferous hornblende schist are believed to

represent original basic intrusives. Granitic sheet and vein intrusions and pegmatite vein

are common, especially in the south of the area. Some degree of granitisation has affected

all the basement system rocks of the area, with the exception of the quartzite and related

rocks.

33..33.. GGeeoollooggyy ooff tthhee IInnvveessttiiggaatteedd SSiittee

The area is overlain by recent deposits of eroded and decayed hard capping of lateritic

iron stone resulting to production of a thick mantle of heavy red soil. Beneath this is high-

level gravel deposits. This is underlain by the Kavirondian Series which consist of the

sediments of felspathic grits, conglomerates, shale and siltstone. The Kavirondian series

has been heavily intruded by a complex of intrusions. These intrusion are granites of

several distinct types. The granites are part of Kitosh Batholith, whose outcrop covers

very considerable tracts of the country north of the Nzoia river. These rocks are usually

porphyritic, coarse-grained, hornblende-biotite granite -the Kisama –Igara granite, which

frequently form characteristic tors, of which Bukaiya and Nyenyese hills are example.



Figure 2: Geological Map of the Study Area (After Beicip- Malmaison – France 1987)

Igara Area

Legend

Rivers

Granitoid Gneisses

Intrusives

Metasediments



44.. HHYYDDRROOGGEEOOLLOOGGYY

The hydrogeology of an area is normally intimately dependent upon the nature of the parent

rock, structural features, weathering processes, recharge mechanism and the form and

frequency of precipitation. The midland environment of Western Kenya area offers a

reasonable groundwater potential. The relatively high rainfall, and the gently peneplain

surface south of Elgon and west of the Nandi escarpment are all favorable characteristics to

ground water occurrence.

44..11.. GGrroouunnddwwaatteerr OOccccuurrrreennccee

Given that suitable storage media exist below ground, the mechanisms by which water

must reach it also affect aquifer potential. Obviously, if no rainfall or riverflow is able to

percolate to a sandy weathered Basement aquifer due to the presence of an aquitard

(impermeable layer) probably clay, the actual potential is very low. If rainfall is low, the

volume of water, which may eventually percolate to a suitable aquifer, is likely to be

relatively small, and possibly mineralized due to high evaporation.

Percolation is dependent on soil structure, vegetation coverage and the erosion state of the

parent rock. Rocks which weather to clayey soils will naturally inhibit percolation (such

as „black cotton‟ soils); conversely, the sandy soils resulting from the erosion of some

Basement System rocks are eminently suited to deep swift percolation.

Recharge is the term applied to the whole mechanism, and includes all the aspects of

parent geology, effective rainfall and percolation. Some aquifer systems are recharged by

water falling a substantial distance away.

Generally there are two types of aquifers that exist within the Basement System rock.

These include those associated with fault and fracture feature and weathered rocks

overlying compact or fresh rock.

44..22.. BBaasseemmeenntt AAqquuiiffeerr OOccccuurrrreennccee

Aquifers associated with fault and fracture features are better than the weathered rock

aquifers overlying compact or fresh rock. These latter aquifers, located in the regolith, are

highly variable in terms of potential yields. This is because the physical transmission of

water within the aquifer body itself is dependent upon the total clay fraction present after

weathering of the parent rock which itself depends on the chemical constitutes of the

parent rock and the mechanism of weathering.

Faults and fractures associated with major movements of the earth‟s crust and often

accompanied by tectonic activity, lead to the formation of coarser material. This, together

with the larger volume of potential storage brought about by the fault, leads to rather

better potential yields, especially where a borehole is sited so as to intersect fault features

at depth.



44..33.. WWeeaatthheerriinngg aanndd iittss SSiiggnniiffiiccaannccee iinn AAqquuiiffeerr OOccccuurrrreennccee

Different zones may be identified in the weathering profile of an area. These are detailed

below.

44..33..11.. TThhee CCoollllaappsseedd ZZoonnee

This may show marked lateral variation but is general sandy on watershed areas with

illuviated clay near the base and sometimes a „stone line‟; on valley slopes, colluvial

material accumulates and in valley bottoms, secondary clay minerals predominate. Slope

bottom laterites may also occur which can result in perched water tables. Permeabilities

vary in accordance with lithology on watersheds. The collapsed zone normally occurs

above the water table.

44..33..22.. SSaapprroolliittee

This is covered by the collapsed zone and is mainly composed of more coarsely

disintegrated and less altered rock material. It is derived by situ weathering from the

bedrock but is disaggregated. Permeability commonly increases at lower levels due to a

lesser development of secondary clay minerals.

44..33..33.. SSaapprroocckk

This is weathered bedrock. It resembles the bedrock zone and is the most important local

aquifer type. Fracture permeability generally increased as a result of weathering (as

compared with fresh bedrock) unless in filled by illuviated clay minerals.

44..33..44.. BBeeddrroocckk

This includes the variably fractured fresh bedrock and the saprock or weathered bedrock.

The saprock-fresh bedrock junction is generally transitional or even fluctuating in banded

sequences. Fracture systems are related either to decompression or to tectonic forces. The

former tend to be sub-horizontal with a decreasing frequency with depth. The latter tend

to be sub-vertical and are often in zonal concentrations. Since tectonic fracturing in

Basement rocks has occurred on several occasions over a long time span and often with

reactivation of old fractures, clear recognition of age or differentiation of shear and tensile

conditions is not to be expected.

Fissure permeability is assumed to correlate to some degree with frequency of fracture

occurrence, with a further assumption that both parameters will decrease with depth.

However, the few detailed studies do not always present comparable features, variations

being probably due to limitations of the database. Fracture distribution probably reflects

fracture systems mainly of decompression type. Fracture sealing in the weathered bedrock

profile is known to occur, probably by clay illuviation, and may result in an increasing



permeability with depth. Fracture occurrence and permeability in the main bedrock may

show an inverse relationship of depth to permeability with large data set. The depth-

permeability relationships at individual borehole sites are obviously affected by the dip of

tectonic fracture zones.

44..33..55.. RReeggoolliitthh

The regolith consists of the collapsed zone and the saprolite. Since weathering is most

effective in the vadose zone and the zone of water table fluctuations, there is a tendency

to develop subdivisions into an upper and lower saprolite relative to current (or previous)

water levels. This is in addition sometimes to a basal brecciated zone where rock

fragmentation is largely unaccompanied by mineralogical changes.

In regions of moderate to high annual rainfall (>600 mm/year), the water table is typically

at shallow levels (<10-15 m) and the regolith aquifer provides the main storage for deep

boreholes as well as both storage and transmissivity for wells and shallow boreholes.

The collapsed zone has developed from the underlying saprolite by further dissolution

and leaching, combined with other formative processes: chemical, physical and

biological. The surface material is typically sandy on watershed areas where these overlie

quartz- rich rocks but changes to sandy clays and clay (montmorrillonite) in valley bottom

lands. The surface sands have high infiltration capacities, which decrease markedly in any

underlying illuviated clay horizons.

Saprolite is derived from in situ weathering and is disaggregated. An upper saprolite may

be distinguished by higher proportions of the more advanced minerals combined with the

earlier forms of secondary clay minerals (smectites). The boundary with the underlying

saprock may be sharp (against coarser-grained, massive rock) or transitional (against

banded or finer-grained rocks)

.

44..44.. HHyyddrrooggeeoollooggyy ooff tthhee IIggaarraa AArreeaa

The type of rainfall is convenctional, which is the major characteristic of Lake Victoria

Basin. The major river is Nzoia river and is very largely drained by its tributaries and sub

tributaries notably Lisumu, Kisama, Viratsi, Siga and Suludhi river. Groundwater

recharge is mainly by direct percolation and precipitation. Groundwater in this area

occurs within the perched water table resting on resistant bands of pegmatitic granite in

pervious weathered biotite granite, weathered and fractured Basement System rocks

overlying the fresh Basement system rock of granitic origin. In places where Basement

rocks are banded, groundwater is expected to be found within weathered or fractured

bands. Recharge is also expected to be via lateral flow over a long distance in well

developed joint system which may offer free passage to water. The weathered zone

immediately below the overburden is saturated and may contain promising amounts of

ground water as a result of damming by other minor intrusives.

It is thus expected that the groundwater potential in the area is medium with the

promising aquifer being the contact zone between the sediments and the underlying



Basement, and also weathered and fractured Basement. In this area groundwater resources

are evenly and widely distributed.

44..55.. BBoorreehhoollee DDaattaa

Table 1: Borehole-C7011 0-2 Lateritic top soil

2-10 Decomposed granite

10-11 Disintegrated granite

11-22 Fractured weathered granites

22-32 Disintegrated granites

32-34 Fresh granite

44..66.. GGrroouunnddwwaatteerr QQuuaalliittyy

From observation of water from the various boreholes and numerous shallow wells in the

Igara area, groundwater quality is generally good in colour, turbidity, odour and taste.

This therefore qualifies the intended uses in the station. However, the client is cautioned

not to use the water for drinking purposes before the water undergoes bacteriological

tests.



55.. GGEEOOPPHHYYSSIICCAALL IINNVVEESSTTIIGGAATTIIOONN MMEETTHHOODDSS

Investigations of the groundwater resources at Igara African Devine Church included the

use of geophysical techniques to probe the sub-surface. A variety of methods are available

to assist in the assessment of geological sub-surface conditions. The main emphasis of the

fieldwork undertaken was to determine the thicknesses and composition of the sub-surface

formations and to identify water-bearing zones.

This information was principally obtained in the field using vertical electrical soundings

(VES) and horizontal resistivity profiling with the TSP Resistivity Meter.

The VES probes the resistivity layering below the site of measurement. This method is

described below.

55..11.. RReessiissttiivviittyy MMeetthhoodd

Vertical electrical soundings (VES) were carried out to probe the condition of the sub-

surface and to confirm the existence of deep groundwater. The VES investigates the

resistivity layering below the site of measurement. This technique is described below.

55..22.. BBaassiicc PPrriinncciipplleess

The electrical properties of rocks in the upper part of the earth's crust are dependent upon

the lithology, porosity, the degree of pore space saturation and the salinity of the pore water.

Saturated rocks have lower resistivities than unsaturated and dry rocks. The higher the

porosity of the saturated rock, or the higher the salinity of the saturating fluids, the lower the

resistivity. The presence of clays and conductive minerals also reduces the resistivity of the

rock.

The resistivity of earth materials can be studied by measuring the electrical potential

distribution produced at the earth's surface by an electric current that is passed through the

earth.

The resistance R of a certain material is directly proportional to its length L and

cross-sectional area A, expressed as:

R = Rs * L/A (in Ohm)

Where Rs is known as the specific resistivity, characteristic of the material and independent

of its shape or size.

With Ohm's Law,

R = dV/I (Ohm)



Where dV is the potential difference across the resistor and I is the electric current through

the resistor. The specific resistivity may be determined by:

Rs = (A/L) * (dV/I) (in Ohm m)

55..33.. VVeerrttiiccaall EElleeccttrriiccaall SSoouunnddiinngg ((VVEESS))

When carrying out a resistivity sounding, current is led into the ground by means of two

electrodes. With two other electrodes, situated near the centre of the array, the potential

field generated by the current is measured.

From the observations of the current strength and the potential difference, and taking into

account the electrode separations, the ground resistivity can be determined.

During a resistivity sounding, the separation between the electrodes is step-wise increased

(in what is known as a Schlumberger Array), thus causing the flow of current to penetrate

greater depths. When plotting the observed resistivity values against depth on double

logarithmic paper, a resistivity graph is formed, which depicts the variation of resistivity

with depth. This graph can be interpreted with the aid of a computer, and the actual

resistivity layering of the subsoil is obtained. The depths and resistivity values provide the

hydrogeologist with information on the geological layering and thus the occurrence of

groundwater.



66.. FFIIEELLDDWWOORRKK AANNDD RREESSUULLTTSS

66..11.. FFiieellddwwoorrkk

Fieldwork was carried out on 30th

September 2012. One vertical electrical soundings (VES),

were done to a maximum A/B=130m.

Table 2: GPS Co-ordinates of the VES Location

VES NO. CO-ORDINATES UTM ALTITUDE

Longitudes Latitudes Latitudes Latitudes

1 E 0340 21‟42.9” N 00

0 31‟ 40.3” 0651558 058356 1836m

66..22.. IInntteerrpprreettaattiioonn RReessuullttss

Interpreted results of the soundings are shown in the table presented below:

Table 3: Interpretation Results for Igara Devine Church

VES

LAYER 1

RE DE

LAYER 2

RE DE

LAYER 3

RE DE

LAYER 4

RE DE

LAYER 4

RE DE

1 36 0.5 111 4 21 27 38 37 1000 >37

RE Resistivity (Ohm-m) DE Depth (m)

The VES interpretation results indicate a shallow superficial layer to a depth of less than 4

m bgl with a resistivity of 36 to111 Ohm-m, comprising dry soils, sandy clays., it is

underlain by a 21 to 38 Ohm m layer to a depth not greater than 37 m, and is interpreted to

be weathered basement rocks which are aquiferous. Below these is underlain by a high

resistivity layer of 1000 Ohm m. This layer comprises of the highly compact Basement

rocks. Water strikes are not expected in this layer.

Table 4: Interpretation Results for Igara Devine Church VES 1

DE

RE

Geological interpretation

0-0.5 36 Dry to sandy clay soils

0.5-4 111 Dry to sandy clay soils

4-27 21 Weathered granodiorites

27-37 38 Weathered granodiorites

> 37 5000 Fresh Basement rocks



Figure 3:Vertical Electrical Sounding Interpretation for Igara VES 1



77.. CCOONNCCLLUUSSIIOONNSS AANNDD RREECCOOMMMMEENNDDAATTIIOONNSS

77..11.. CCoonncclluussiioonnss FFiieellddwwoorrkk

The investigated area is located in a hydrogeological zone which is characterized by

medium groundwater potential. Aquifers encountered between 10 and 60 metres are

expected to yield about 1to 5 m3/hr of water for domestic purposes. Deeper aquifers are

not expected below this depth.

Water strikes at moderate depths may occur, due to probably tapping of perched water

tables resting on resistant bands of pegmatitic gneiss in pervious weathered biotite gneiss;

the granites in the area might have a well developed joint system than usual, which may

be assumed to afford free passage to water or between the regolith and the fresh

Basement rock.

77..22.. RReeccoommmmeennddaattiioonnss

In view of the above it is recommended that:

- A shallow Well be excavated to a depth of 20-25 m bgl

- If the shallow Well is not very productive then a Borehole be drilled

- As an alternative an 8" diameter borehole be drilled to a depth of about 60 m. The

yield will be in the order of 1 to 5 m3/hr.

- The borehole should be installed with high quality uPVC casings and screens.

- A monitoring piezometer should be installed in the borehole to enable monitoring

of the water level in the well.

- A master meter should be installed to record the amount of water abstracted from

the borehole.

- The most suitable location for the Shallow Well/Borehole was shown to the Mr.

William Ashitiva, Mr. George Wafula and M/s Mary Opech all residents and

church members of the African Devine Church Igara.

Appendix 1 gives additional recommendations on the construction and completion of a

borehole.



88.. RREEFFEERREENNCCEESS

A.B GIBSON, 1952,Geology of the Broderick Falls Area

C STANSFIELD HITCHEN, 1937,Geological Survey Of Kenya of No.2 Mining

Area, - KAVIRONDO

JONES M J., 1985, The Weathered Zone Aquifers of the Basement Complex

areas of Africa. Q.J eng. Geol. London. Vol. 18 pp 35-46.



APPENDICES



Appendix 1:Drilling

Drilling Technique

Drilling should be carried out with an appropriate tool - either percussion or rotary

machines will be suitable, though the latter are considerably faster. Geological rock samples

should be collected at 2 metre intervals. Struck and rest water levels and if possible,

estimates of the yield of individual aquifers encountered, should also be noted.

Well Design

The design of the well should ensure that screens are placed against the optimum aquifer

zones. The final design should be made by an experienced hydrogeologist.

Casing and Screens

The well should be cased and screened with good quality material. Owing to the shallow

depth of the boreholes, it is recommended to use uPVC casings and screens of high open

surface area.

We strongly advise against the use of torch-cut steel well-casing as screen. In general, its

use will reduce well efficiency (which leads to lower yield), increase pumping costs through

greater drawdown, increase maintenance costs, and eventually reduction of the potential

effective life of the well.

Gravel Pack

The use of a gravelpack is recommended within the aquifer zone, because the aquifer could

contain sands or silts which are finer than the screen slot size. An 8" diameter borehole

screened at 6" will leave an annular space of approximately 1", which should be sufficient.

Should the slot size chosen be too large, the well will pump sand, thus damaging the

pumping plant, and leading to gradual `siltation' of the well. The slot size should be in the

order of 1.5 mm. The grain size of the gravel pack should be an average 2 - 4 mm.

Well Construction

Once the design has been agreed, construction can proceed. In installing screen and casing,

centralizers at 6 metre intervals should be used to ensure centrality within the borehole.

This is particularly important for correct insertion of artificial gravel pack all around the

screen. After installation, gravel packed sections should be sealed off top and bottom with

clay (2 m).



The remaining annular space should be backfilled with an inert material, and the top five

metres grouted with cement to ensure that no surface water at the well head can enter the

well bore and cause contamination.

Well Development

Once screen, pack, seals and backfill have been installed, the well should be developed.

Development aims at repairing the damage done to the aquifer during the course of drilling

by removing clays and other additives from the borehole walls. Secondly, it alters the

physical characteristics of the aquifer around the screen and removes fine particles.

We do not advocate the use of overpumping as a means of development since it only

increases permeability in zones which are already permeable. Instead, we would

recommend the use of air or water jetting, or the use of the mechanical plunger, which

physically agitates the gravel pack and adjacent aquifer material. This is an extremely

efficient method of developing and cleaning wells.

Well development is an expensive element in the completion of a well, but is usually

justified in longer well-life, greater efficiencies, lower operational and maintenance costs

and a more constant yield. Within this frame the pump should be installed at least 2 m

above the screen, certainly not at the same depth as the screen.

Well Testing

After development and preliminary tests, a long-duration well test should be carried

out. Well tests have to be carried out on all newly-completed wells, because apart from

giving an indication of the quality of drilling, design and development, it also yields

information on aquifer parameters which are vital to the hydrogeologist.

A well test consists of pumping a well from a measured start level (Water Rest Level -

(WRL) at a known or measured yield, and simultaneously recording the discharge rate and

the resulting drawdowns as a function of time. Once a dynamic water level (DWL) is

reached, the rate of inflow to the well equals the rate of pumping. Usually the rate of

pumping is increased step wise during the test. The results of the test will enable a

hydrogeologist to calculate the optimum pumping rate, the pump installation depth, and the

drawdown for a given discharge rate.



NB: Not to scale

Groundlevel

Cement grout

Inert backfill

Bentonite seal

2-4 mm Gravel pack

Bottom cap

Groundlevel

Concrete slab Well cover

Plain casing

Sanitary casing

Screens

Schematic Design for Borehole completion



Schematic Design for Borehole Completion

hydrogeological & geophysical investigations · hydrogeology and existing borehole data, for...

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